Many fuel injection systems for internal combustion engines use a fuel distributor for delivering fuel to fuel injectors associated with respective engine cylinders. The distributor functions to fluidically connect a supply fuel passage to each fuel injector or engine cylinder one at a time through separate fuel injection lines extending from the distributor to each injector. The conventional rotary distributor includes a rotor shaft rotatably mounted in a bore formed in a distributor housing. The shaft typically includes a radial supply passage and a radial fuel outlet port, axially spaced from the supply passage, and fluidically connected with one another by means of an axial passage in the shaft. The distributor housing includes a plurality of fuel distribution passages having ports evenly spaced around the circumference of the bore. During shaft rotation, the outlet port sequentially aligns or registers with the fuel delivery passages to permit fuel delivery to each engine cylinder. Each period of time defined by the alignment of the outlet port and a delivery passage creates a window of opportunity for injection, during which a controlled quantity of fuel is delivered through the aligned passages to an injector associated with a given engine cylinder. During operation, the shaft is operatively connected to the engine crankshaft, which provides the power for continuous rotation.
Although the conventional rotary distributor operates adequately to create sequential windows of opportunity for delivering metered quantities of fuel, certain problems exist when a rotary distributor is used in a very high pressure fuel system capable of achieving pressures in excess of approximately 15,000 psi. Recent and upcoming legislation resulting from a concern to improve fuel economy and reduce emissions continues to place strict emission standards on engine manufacturers. In order for new engines to meet these standards, it is necessary to produce fuel injection systems capable of achieving higher injection pressures while maintaining accurate and reliable control of the metering and timing functions. One such high pressure fuel injection system is disclosed in PCT Publication No. WO 94/27041 entitled Compact High Pressure Fuel System with Accumulator, commonly assigned to the assignee of the present application, wherein an accumulator temporarily stores fuel supply by high pressure variable displacement pump for delivery to a distributor via a solenoid controlled three-way valve. This system is capable of achieving extremely high injection pressures in excess of 20,000 psi. This rotary distributor relies on a clearance fit between the rotor and the bore, and very short sealing lengths, to isolate the high pressure injection occurring in one delivery passage from the remaining passages. At high injection pressures, fuel leakage through the clearance between the shaft and bore becomes unacceptable. More importantly, the extremely high pressure fuel existing in the clearance gap or region surrounding the outlet port formed in the rotor, acts on the surface of the rotor to create a side load or force tending to move the rotor toward the opposite side of the bore. One or more unpressurized ports formed in the distributor housing on the opposite side of the bore interrupt the bearing surface which is used to support the side load. As a result, the fuel between the bearing surface and the rotor is squeezed out by the side loading, causing bending of the rotor and contact between the rotor and the bore surface and ultimately resulting in seizure of the rotor in the bore. The severity of the side loading is increased as the injection pressure, and injection event duration, are increased as required by modern fuel systems. Another important problem experienced in conventional rotary distributors is thermal seizure. The fuel in the clearance region between the rotor and bore experiences excessive heating during operation due to high pressure leakage, flow restriction, compression, friction due to rotor-to-housing contact, and the collapsing of vaporized fuel, i.e. cavitation. Of course, the aforementioned increased side loading causes higher heat loads from contact and leakage. Even if the side loading problem is minimized, the heat load increases with increasing injection pressure (more fuel spilling during depressurization thus increasing leakage and cavitation) and decreasing fueling amount (more pressurization/depressurization cycles for a given fuel volume). In conjunction with cavitation heat loads, heating occurs in the surface passages on the rotor due to fuel spilling during depressurization. This localized heating can cause local expansion of the rotor resulting in a press-fit between the rotor and opposing housing wall thereby resulting in seizure.
U.S. Pat. No. 5,619,971 issued to Kubo et al. discloses a rotary distributor including a pressure balancing feature for balancing the fluid pressure forces acting on the rotor so as to prevent seizure. In a conventional manner, the rotor includes an axial supply passage which supplies high pressure fuel to a radial passage leading to a distribution port formed in the outer surface of the rotor. The pressure balancing feature includes two ports connected to the axial supply passage and positioned in such a manner that is not offset in the direction of the axis from the opening of the distribution port. The ports are formed so as to be offset symmetrically in the direction of the circumference of the rotor relative to the distribution port. The ports are formed in such a manner that they will not communicate with any of the delivery passages formed in the distributor housing when the distribution port is in communication with any one of the delivery passages. In addition, the total of the opening areas of the ports is approximately equal to the opening area of the distribution port. Thus, during an injection event, high pressure fuel is supplied to the distribution port and the pressure balancing ports. Kubo asserts that the pressure force on the rotor developed at the distribution port is cancelled out by the total of the forces applied by high pressure fuel acting at the opposite ports thereby balancing the pressure and preventing rotor seizure. Kubo et al. also suggests forming flow passages in the outer surface of the rotor for directing high pressure fuel to the delivery passages formed in the distributor housing between injection events and subsequently draining fuel from the passages prior to the next injection event for the particular delivery passage. Thus, these passages are used to supply cooling and lubricating fuel to the clearance gap between the rotor and housing bore.
Although the Kubo et al. distributor may function adequately to distribute fuel in certain applications, the Kubo et al. distributor does not effectively reduce side loading nor effectively cool the distributor rotor sufficiently in many high pressure applications. Specifically, the ports used to balance the distribution port pressure forces do not create a totally pressure balanced rotor thereby resulting in excessive side loading at high injection pressures. Also, the coolant flow path is merely provided intermittently during operation and therefore fails to adequately cool the fuel in the clearance gap between the rotor and opposing housing wall, and the rotor and housing surfaces. Moreover, the Kubo et al. coolant system uses fuel having an undesirably high temperature.
Consequently, there is a need for an improved rotary fuel distributor capable of effectively and reliably distributing high pressure fuel to the cylinders of an engine while minimizing side loading of the distributor rotor and high temperatures thereby avoiding excessive rotor wear and rotor seizure.